Phase 6 is the last major phase of our program. The key difference that warrants a new major phase is the extreme distances involved. This mostly breaks the ability for timely delivery to and from the region, and round trip communication with the Solar System is measured in years to decades. Expansion of civilization to these regions would require high self-sufficiency in transport and good enough production systems to enable growth without much assistance. It is far enough in the future that we can only speculate about development in very general terms. We currently divide this major phase into three parts: 6A, 6B, and 6C. The first two cover the interstellar regions between stars to a distance of 20 light-years (LY), and stellar systems, including the regions their gravity dominates, to the same distance. The third covers everything beyond 20 LY, but for now that is mainly a place-holder. This section gathers our early ideas about these three phases, pending further concept exploration work.

Interstellar space, the cold regions between stars, is not much different from the environment of the outer parts of the Oort Cloud in Phase 4F. We know very little as yet about equivalent cometary clouds around other stars, or free-floating objects not attached to stars. We have better information about planets and dust disks around other stars. Their parent stars tell us where to look, and the stars themselves provide some data about the planets from Doppler shifts and transits. Disks are visible mainly in the infrared, and are found around younger stars. The number of discovered planets is growing rapidly, from none before 1988 to 3500 confirmed and 4500 candidates by the end of 2017. We expect there are also many smaller objects in systems with planets, but today we can only infer them indirectly.

A future method to inventory these smaller objects is using the Sun as a gravity lens. It brings objects into useful focus around 800 AU from us, in the Scattered Disk region. Placing telescopes directly opposite a star of interest would allow much more detailed observations than otherwise possible, because the 2 million km effective diameter of the lens allows extremely high resolution. Putting instruments at such high distance from Earth and getting data back will not happen until Phase 4F, which is well in the future. Since even making an inventory of what resources are available requires advances in technology, plans to use those resources for development are even further off. They will depend on experience gained in earlier phases and require great improvements in transportation methods.

The Nearby Interstellar region is the next in distance after the Oort Cloud portion of Phase 4F. It begins with orbits whose semi-major axis a = 100,000 AU or more from the Sun, where our star's gravity is no longer dominant. For design purposes we set an arbitrary outer boundary of 20 LY from the Sun. If we can gather supplies and rebuild our equipment at that distance, then later projects can travel farther in increments of 20 LY using the same designs. The 40 LY diameter sphere surrounding our Sun makes up the nearby portion of the Solar Neighborhood (Bovy, 2017), which extends to 250 LY. This in turn is a small is part of the 100,000+ LY diameter Milky Way galaxy we belong to. We are about half-way from the core to the rim of the galaxy, near the central plane.

This phase covers the spaces between stars, so it excludes star systems, the objects which orbit them, and the orbital regions of gravitational dominance surrounding them. The Phase 6A region's volume therefore resembles the solid portion of Swiss or Emmentaler cheese (Figure 4.n-5), with holes around each star system not included. Natural or artificial objects which are moving fast enough they are not tied to any star, and are more than half the region boundary from any star (i.e. >50,000 AU in the case of the Sun) are counted as interstellar. Otherwise they are considered temporary members of a stellar system.

Figure 4.n-5 - Emmentaler cheese. The solid portion represents the volume of the Nearby Interstellar region. The holes represent the regions around stars, which are assigned to Phase 6B.

The open space between stars includes a number of components. The most massive of these include Sub-Brown Dwarfs, which formed the same way as stars by the collapse of a gas cloud. They do not have enough mass for deuterium fusion, and are therefore not stars. Their masses range from 1-13 times Jupiter's (MJ). The lower bound is set by not having enough mass to collapse, and the upper bound by sufficient mass to initiate fusion, making them stars. Several such free-floating objects have been detected. They do not orbit a larger brown dwarf or regular star. The other route to forming large interstellar objects is from a planetary system which forms around around a star. Later gravitational interactions can eject some of the objects into interstellar space. The largest lost objects are called Rogue Planets. Their mass can range from the same upper limit as sub-brown dwarfs (13 MJ) down to a lower limit where they don't have enough mass to assume a round shape. This is somewhat below 1000 km in diameter, depending on composition, at which point they are no longer considered planetary size. Rogue planets are distinguished by having a higher concentration of heavier elements than sub-brown dwarfs. This is due to more of the heavier elements tending to condense into planets, and the lighter ones tending to be blown away by the parent star.

Simulations of the history of the Solar Nebula (Shannon, 2014) indicate that about 80% of the original small bodies within 40 AU of the Sun were ejected into interstellar space. With over 3,500 confirmed Exoplanets by late 2017, we now know that formation of planetary systems is common around stars (see NASA Exoplanet Archive). So if the same ejection process happened for other stellar systems, then interstellar space should be filled with a large population of objects from many stars. The largest of these objects would qualify as the rogue planets previously mentioned, but their size distribution should continue down to dust-sized particles. We define dust particles as those less than 1 mm in size, and Meteoroids as those from 1 mm to 1 meter. Between 1 meter and planetary size, we class them as comets if they are icy, and asteroids if they are rocky. The different compositions come from where they originally formed around a star, and later events in their history.

Only one sub-planetary Interstellar Object has been discovered so far, in late 2017. 1I/'Oumuamua is only about 160 meters in size, and apparently of rocky composition. By chance it happened to be 0.2 AU from Earth when discovered, making it close and bright enough to be detected. It is moving 26 km/s above solar escape velocity, so was never bound to the Sun. We count it as a temporary member of our Solar System for the next 9,000 years, until it reaches 50,000 AU distance. Due to the relative velocities of their parent stars, and the age of the Milky Way, objects like this one, that are currently within 20 LY, could have started anywhere in the galaxy, and even from outside it.

A few of the most massive non-stellar objects, in the Sub-Brown Dwarf range, have been detected by their infrared glow. Smaller objects will have rapidly cooled to ambient interstellar temperatures, and would have nearly no light reflected from nearby stars, making them extremely difficult to detect with current instruments. Therefore the population of these smaller objects is nearly unknown at present, and only roughly estimated from losses by our own Solar System. A possible future method for finding them is to use natural gravity-focused light from stars, or artificial lasers, to scan around a region looking for reflections. Moving the scanner along interstellar paths would then build up a map of object locations. More investigation of this concept is needed to determine if it is feasible, and other detection methods should be pursued.

Figure 4.n-6 - The Motion of the Sun and nearby interstellar clouds.

In addition to the objects larger than 1 mm, the Interstellar Medium between stars contains gas, dust, charged particles, magnetic fields, and electromagnetic radiation. The density and temperature of the medium varies by location. There is also the hypothetical Dark Matter and Dark Energy. We don't yet understand what the "dark" components are or how to use them. They are of scientific interest, but they can be ignored as far as our program is concerned. The Sun is presently moving through a region of slightly higher gas density called the Local Interstellar Cloud (Figure 4.n-6). It will continue to do so for the next 10-20,000 years. The local cloud has a gas density of about 0.3 atoms per cubic centimeter, or 1 gram per 564 km cube. This does not include other components of the interstellar medium, or any larger objects that may be present.

Stellar energy sources are too small in the region for practical use, except possibly along lines of gravitationally focused starlight. Ambient temperatures will mostly be close to the cosmic background temperature of 2.7K. Travel time is many years with known technology, and depends on future improvements to reach useful engineering time scales. Round-trip communication time will range from 3 to 40 years from Earth, and up to 80 years across the region. Stellar radiation is generally not a factor in this region, but cosmic radiation still is.

We don't yet know enough about the material and energy resources in the region to do more than speculate about development projects. We think it is possible the region will be used for fast travel by self-contained vehicles on their way to other star systems, or for slow travel by permanent colonies, who use local resources as they go. Stationary locations don't mean much in a region where everything from stars to gas clouds are in motion relative to each other. Even if you stay put relative to the Sun, other things will still move past you. The great distances from the Sun and other stars are likely detach interstellar industry from regular trade with the rest of civilization. Science, exploration, and seeding interstellar colonies are possible future activities.

Production

Concepts for mining materials from the region are deferred for now, until better data is available on what is present. The production functions we consider for now are aboard transport vehicles and colonies. Trips with even advanced transportation will take years. So fast vehicles will still likely need to do maintenance and repair, either from spare parts and supplies, or to produce new items from wastes and scrap. These technologies should have been developed in the previous phases around the Sun. Permanent interstellar colonies don't reside around one star. They travel between stars, and either stop to gather supplies, or collect them while in motion. Such colonies may be planted on or around existing large bodies, and mine them for resources. Changing the course of a large body would be difficult, so the colony would have to accept the existing trajectory, migrate to a different body, or spawn new colonies which then follow their own courses.

By their nature, isolated interstellar colonies would have to produce most of what they need locally, with perhaps some deliveries by fast transport. The first such colonies would be built somewhere in the Solar System, then placed on their chosen course once functioning. They may evolve from Oort Cloud colonies, whose environment is not significantly different from interstellar space. Any type of production or other function will require energy. Two possibilities are gravity-focused beamed power from local stars, and nuclear energy, either fission or fusion.

Habitation

Habitation designs should not be significantly different from those developed for the outer regions of the Solar System, since the environments are similar.

Transport

Interstellar transport can be divided into slow and fast types. The slow type is on the order of stellar velocities (5-500 km/s) using large habitats with large material reserves and nuclear or beamed energy sources. These habitats can potentially mine cometary clouds around stars and rogue objects between stars. When they get close enough to a selected star, they can enter orbit and travel with it, either permanently or later set course for a new destination. At any time, such habitats may build and spawn additional ones. Travel times between stars at slow interstellar speeds would be 3000 years or longer.

Fast interstellar is dominated by the higher energy required to reach velocities greater than 500 km/s, and shorten the time to a destination. Possible transport methods are discussed in Part 2 of this book. High energy candidates include fusion powered engines and beamed power from local stars. Rather than a large habitat with a full range of civilized activity, fast vehicles operate more like ships on Earth, with a crew dedicated to reaching a destination and maintaining operations.

Services

Possible future services located in interstellar regions include science, exploration, and communications relay.

Based on past exposure to fictional works, most people assume that a "starship" will be a futuristic shape with big engines on the back. Instead, imagine colonizing a long period comet, one of the ones that came from the Oort cloud, and is heading back out there. Comets are made of a mix of ices (water, methane, ammonia, CO2, etc) and rocky materials. If there are not enough metals, then a metallic asteroid can be matched to its orbit. You then build your colony mostly out of the materials already there. Comets range in size up to 50 km in diameter, and there are a large number of outer system bodies more than 100 km in diameter. A 50 km comet contains about a 1500 year supply of Earth's total mining output, which should be more than enough to sustain a colony with recycling.

The Oort cloud is many times the distance of the Earth from the Sun, and the velocity needed to get the comet to leave the Sun and head for another star is very small. All the ices have some amount of hydrogen, and thus deuterium, which means if you know how to build fusion reactors, you have power for a very long time. Rather than a sleek ship, your "vehicle" is a city attached to or built around a comet core, which over time is converted into necessary items for the colonists. It will be a long trip, but you have a large amount of space to live in, and the colonists can make occasional side trips to other comets as they go past to get additional supplies.

There are an estimated trillion comets in the Oort Cloud around the Sun, and some will be along your existing route, more or less. The average spacing is something like 6 AU, about the distance to Jupiter. At the slowest interstellar velocity, 5 km/s, you would pass one per 6 years on average. So there are plenty of mining opportunities, and you can in theory seed other comets as you pass by with new colonies. If some people feel like it, they could head back to the Sun, the velocities are low enough to do that. When you reach the edge of the Oort Cloud region, you can continue this sedate interstellar travel by using objects between the stars, and the cometary clouds around other stars. The requirements for this kind of slow star travel include fusion power, and knowing how to build permanent habitats in space. Both of these should have been developed in previous phases. Another requirement is a way to detect small objects along the travel path, in the absence of starlight to illuminate them.

Starlines

Gravitational lensing occurs around every massive object. In fact, measuring the bending of light during a solar eclipse 100 years ago was the first proof of relativity theory. For the Sun, the light bent from all sides comes to a focus at distances greater than ~540 AU. The focus is not to a point, but rather a radial line. This is because photons that miss the edge of the Sun by a larger distance are bent less, and thus focus farther away. Every star in the sky therefore produces a line of focused light on the other side of the Sun, and so we call them Starlines. Every other star in the interstellar region also produces a pattern of starlines surrounding it. This forms a network of lines of light filling interstellar space. If the starlight is sufficiently well focused, it may prove useful for interstellar power and propulsion.

Interstellar travel and planets around other stars have been explored in Science Fiction for a number of decades. The authors of such works can assume whatever transportation methods and planetary environments are needed for their stories. Engineers considering development projects can draw their ideas from fiction, but are limited to actual technologies and real places to implement their plans. This phase of our program covers development of the regions around stars other than the Sun. It logically follows Phase 6A, which is concerned with the regions between stars, since we must travel through such regions to reach other stars. It also follows all the earlier phases which are dedicated to developing various parts of our Solar System. Like Phase 6A, we limit this one arbitrarily to within 20 light-years of the Sun. This is a large enough sample of stars and their attendant systems to identify design requirements for them, and is a large enough range of distances to identify whatever problems that will cause. Development at distances beyond 20 LY are reserved to the last current program phase, 6C - Farther Interstellar.

For our purpose, we identify as stars any object large enough to undergo nuclear fusion, past, present, or future. This includes brown dwarfs who only undergo deuterium fusion, larger stars that mainly undergo hydrogen fusion, and stellar-mass remnants like the six White Dwarfs which are closer than 20 LY. A stellar system may include one or more stars bound by gravity, and all the attendant objects and material which orbits them. We define an orbital region surrounding each stellar system extending from the center of mass, to orbits with a semi-major axis of 100,000 AU times the square root of the system mass in units of the Sun's mass. This is a region where the local system gravity dominates the rest of the surrounding galaxy, and they are able to retain cometary clouds. If two systems are near each other the orbital regions may overlap. In that case we draw a boundary between them where their gravity is equal. The stellar systems and their orbital regions are bubbles set within the larger Nearby Interstellar region filling the space between them. We separate Phases 6A and 6B because stellar systems have more in common with the earlier phases around our Sun than the spaces between stars.

Possibly the most significant feature of the region is the stellar systems which make it up are all in relative motion to each other, with an average velocity of 50 km/s relative to the Sun. This motion is in addition to the general rotation of the Milky Way galaxy, which is about 225 km/s in this area. There are about 105 Nearby Systems within 20 light years, including our Sun. This list should be nearly complete. Given their average velocity, they will take 120,000 years to travel 20 light-years, so the nearby population will change about every 1150 years on average. Current transport methods require much more time than this to reach 20 light-years. So future plans for the region should take into account the motions of the stars and changes in the nearby population.

Figure 4.n-7 - Number of known exoplanets by size as of 2014.

As of 2017 there are about 31 confirmed exoplanets outside of our Solar System, and within the 20 LY limit. Our detection methods are still biased towards larger planets (Figure 4.n-7), with nearly all confirmed planets larger than the Earth's mass. The size distribution in our own Solar System indicates the number of objects increases as their size decreases. This makes it likely that more smaller planet, and many more objects smaller than planets exist around other stars and await discovery. We expect the count of nearby planets to increase over the next few years as our instruments improve. There are also two known nearby Circumstellar Disks around Epsilon Eridani and Tau Ceti, which are detectable by their dust component. It is likely comets and asteroids and some larger objects also exist around these stars but are not yet detected. Those would be in addition to the Earth-mass or heavier planets already discovered or suspected.

We have a reasonably complete list of stars to 20 LY, because they are bright and nearby. We don't yet have a complete list of planets and know nothing of smaller bodies aside from dust particles when they form disks. Therefore concepts for developing the Nearby Exostellar region must remain speculative and preliminary at this time.

Production

As mentioned in the introduction to Phase 6, we would like to observe nearby stars and their surroundings by using the Sun as a giant gravitational lens. This should enable us to inventory their resources in much more detail than near-term telescopes. Following that would be sending smaller robotic probes to more closely examine whatever is found around these stars. Once sufficient information is available we can plan how to start production in the region. Self-expanding production has been a theme throughout our program, and should be well developed by this phase. So we would expect to also use that approach around other stars. An open question is whether to start production with a robotic seed factory sent ahead of people, or begin with a larger system that arrives with the first people and is already supporting them. We probably can't answer that question until more experience is gained around our home star.

Habitation

Habitats for people and other living things should be mostly similar to those developed for regions around the Sun. Differences in temperature, radiation levels, and stellar spectra may require some modifications to previous designs.

Transport

Transport between stars is covered above under Phase 6A. Travel within a given stellar region should be able to use similar technologies as developed around the Sun. One difference is the brightness and temperatures of stars vary, so stellar energy based transport would need modification to account for this.

Services

Due to extreme distance, the service activities we see for now are confined to science, exploration, and seeding independent colonies.

Our last defined phase is a place-holder to cover the remainder of the accessible Universe. It begins 20 LY from the Sun, and extends as far as transport methods make it possible to reach. Current and near-term transport is very far from able to reach such distances. So the only work we assign to this phase for now is transport improvements. Other work is reserved to some point in the future. This includes defining additional phases if the need arises.